Olefins are traditionally produced from petroleum feedstock by catalytic or steam cracking processes. These cracking processes, especially steam cracking, produce prime olefins such as ethylene and/or propylene from a variety of hydrocarbon feedstock. Ethylene and propylene are important commodity petrochemicals useful in many processes for making plastics and other chemical compounds. Ethylene is used to make various polyethylene plastics, and in making other chemicals such as vinyl chloride, ethylene oxide, ethylbenzene and alcohol. Propylene is used to make various polypropylene plastics, and in making other chemicals such as acrylonitrile and propylene oxide.
The petrochemical industry has known for some time that oxygenates, especially alcohols, are convertible into prime olefins. This process is referred to as the oxygenate-to-olefin process. The preferred oxygenate for prime olefin production is methanol. The process of converting methanol to olefins is called the methanol-to-olefins process.
There are numerous technologies available for producing oxygenates, and particularly methanol, including fermentation or reaction of synthesis gas derived from natural gas, petroleum liquids, carbonaceous materials including coal, recycled plastics, municipal waste or any other organic material. The most common process for producing methanol is a two-step process of converting natural gas to synthesis gas. Then, synthesis gas is converted to methanol.
Generally, the production of synthesis gas involves a combustion reaction of natural gas, mostly methane, and an oxygen source into hydrogen, carbon monoxide and/or carbon dioxide. Synthesis gas production processes are well known, and include conventional steam reforming, autothermal reforming or a combination thereof.
Synthesis gas is then processed into methanol. Specifically, the components of synthesis gas (i.e., hydrogen, carbon monoxide and/or carbon dioxide) are catalytically reacted in a methanol reactor in the presence of a heterogeneous catalyst. For example, in one process, methanol is produced using a copper/zinc oxide catalyst in a water-cooled tubular methanol reactor.
The methanol is then converted to olefins in a methanol-to-olefins process and produces a reactor effluent stream. The reactor effluent stream contains desirable olefin product as well as byproducts. The byproducts are typically removed from any olefin product stream to make acceptable purity grades of olefin stream. Two very similar byproducts are methyl acetylene and propadiene. Methyl acetylene and/or propadiene (optionally referred to collectively as “MAPD”) present a particular challenge for removal because they have similar relative volatility to propylene.
MAPD is destroyed by a hydrogenation reaction. Hydrogenation converts MAPD to propylene or propane. Hydrogenation reactors are not useful when it is desirable to recover the MAPD. Additionally, hydrogenation reactions are exothermic and carry a certain level of safety risk. Removing MAPD without the use of an exothermic reactor would be advantageous in certain instances.
U.S. Pat. No. 6,066,238 teaches a method of separating MAPD from an olefin stream. The first step provides an olefin stream comprising propylene and MAPD that optionally contains less than 1 wt. % C4 hydrocarbon stream. Propane is added to the olefin stream. Then, the propane containing olefin stream is fractionated to produce an overhead stream comprising propylene and a bottoms fraction comprising propane and no more than 25 wt. % MAPD. The MAPD is then extracted from the propane with dimethyl formamide.
It would be desirable to separate MAPD from a propylene stream in a safe and effective way that avoids chemically converting the MAPD. The present invention satisfies these and other needs.